Selective hydrogenation of conjugated diolefin poylmers

ABSTRACT

This is an improvement in a process for the hydrogenation of conjugated diolefin polymers which comprises polymerizing or copolymerizing at least one conjugated diolefin with an organo alkali metal polymerization initiator in a suitable solvent thereby creating a living polymer, terminating the polymerization by the addition of hydrogen, thereby generating an alkali metal hydride, and effecting selective hydrogenation of the unsaturated double bonds in the conjugated diolefin units of the terminated polymer by contacting the polymer, in the absence of hydrocarbon lithium and alkoxy lithium compounds, with hydrogen in the presence of at least one bis(cyclopentadienyl)titanium compound. The improvement comprises treating the polymer, before hydrogenation, with a reagent to increase catalyst activity, the reagent being such that it will react with the alkali metal hydride present in the polymer.

BACKGROUND OF THE INVENTION

The present invention relates to an improved process of the productionof selectively hydrogenated polymers of conjugated dienes and moreparticularly to such a process utilizing a titanium hydrogenationcatalyst.

U.S. Pat. No. 5,291,990 describes a process for the hydrogenation ofconjugated diolefin polymers which first involves the polymerization ofcopolymerization of such monomers with an organo alkali metalpolymerization initiator in a suitable solvent thereby creating a livingpolymer. The living polymer is terminated by the addition of hydrogen.Finally, selective hydrogenation of the unsaturated double bonds in theconjugated diolefin units of the terminated polymer is carried out inthe presence of at least one bis(cyclopentadienyl)titanium compoundpreferably of the formula: ##STR1## wherein R₁ and R₂ are the same ordifferent and are selected from the groups consisting of halogens, C₁-C₈ alkyl and alkoxyls, C₆ -C₈ aryloxys, aralkyls, cycloalkyls, silylsand carbonyls. The hydrogenation step is carried out in the absence ofhydrocarbon lithium and alkoxy lithium compounds.

We have found that the above process, while being extremely advantageousfor the hydrogenation of most polymers, has some disadvantages. We havediscovered that for low molecular weight polymers, i.e. molecularweights (true peak molecular weight determined by gel permeationchromatography) below about 100,000 and/or where the polymerconcentration in solution is high, the above process may produce anexcess of lithium hydride which appears to be detrimental to thesuccessful hydrogenation because an excess of alkali metal, usuallylithium, hydride (LiH) may aid in the ultimate destabilization of thehydrogenation catalyst. This problem is most evident when low levels ofbis(cyclopentadienyl)titanium compounds are used. This problem couldalso occur when higher molecular weight polymers are hydrogenated whensuch polymers are in solution in higher than the normal concentrations(i.e. more than about 15 to about 35%).

SUMMARY OF THE INVENTION

The present invention is an improvement upon the process describedabove. The polymer solution containing the alkali metal hydride, usuallyLiH, is reacted with a reagent. Generally, only a small amount of thereagent is needed to increase hydrogenation catalyst activity. It isvital that only the excess hydride be reacted with the reagents. If toomuch hydride is reacted, the catalyst system will not be effective inhydrogenating the polymer. The hydride:Ti molar ratio should not bereduced to below about 6:1.

DETAILED DESCRIPTION OF THE INVENTION

As is well known, polymers containing both aromatic and ethylenicunsaturation can be prepared by copolymerizing one or more polyolefins,particularly a diolefin, by themselves or with one or more alkenylaromatic hydrocarbon monomers. The copolymers may, of course, be random,tapered, block or a combination of these, as well as linear, star orradial.

As is well known, polymers containing ethylenic unsaturation or botharomatic and ethylenic unsaturation may be prepared using anionicinitiators or polymerization catalysts. Such polymers may be preparedusing bulk, solution or emulsion techniques. In any case, the polymercontaining at least ethylenic unsaturation will, generally, be recoveredas a solid such as a crumb, a powder, a pellet or the like. Polymerscontaining ethylenic unsaturation and polymers containing both aromaticand ethylenic unsaturation are, of course, available commercially fromseveral suppliers.

In general, when solution anionic techniques are used, conjugateddiolefin polymers and copolymers of conjugated diolefins and alkenylaromatic hydrocarbons are prepared by contacting the monomer or monomersto be polymerized simultaneously or sequentially with an anionicpolymerization initiator such as Group IA metals, their alkyls, amides,silanolates, napthalides, biphenyls and antrhacenyl derivatives. It ispreferred to use an organoalkali metal (such as sodium or potassium)compound in a suitable solvent at a temperature within the range fromabout -150° C. to about 300° C., preferably at a temperature within therange of about 0° C. to about 100° C. Particularly effective anionicpolymerization initiators are organolithium compounds have the generalformula:

    RLi.sub.n

Wherein:

R is an aliphatic, cycloaliphatic, aromatic or alkyl-substitutedaromatic hydrocarbon radical having from 1 to about 20 carbon atoms; and

n is an integer of 1 to 4.

Conjugated diolefins which may be polymerized anionically include thoseconjugated diolefins containing from 4 to about 12 carbon atoms such as1,3-butadiene, isoprene, piperylene, methylpentadiene, phenylbutadiene,3,4-dimentyl-1,3-hexadiene, 4,5-diethyl-1,3-octadiene and the like.Conjugated diolefins containing from 4 to about 8 carbon atoms arepreferred for use in such polymers. Alkenyl aromatic hydrocarbons whichmay be copolymerized include vinyl aryl compounds such as styrene,various alkyl-substituted styrenes, alkoxy-substituted styrenes, 2-vinylpyridine, 4-vinyl pyridine, vinyl naphthalene, alkyl-substituted vinylaphthalenes and the like.

The polymers which can be used in accordance with the process of thisinvention include all of those described in the aforementioned U.S. Pat.No. 5,291,990 which is herein incorporated by reference. In theproduction of all of these, the polymerization is terminated byutilizing hydrogen gas in place of the conventionally used alcoholterminating agent. The living polymer, of more accurately, the livingend of the polymer chain, is terminated by the addition of hydrogenthereto. The theoretical termination reaction is shown using anstyrene-butadiene-styrene block copolymers for exemplary purposes:

    S-B-S-Li.sup.+ +H.sub.2 →S-B-SH+LiH

As shown above, it is theorized that lithium hydride is formed duringthe termination process. Formed in this manner, it is not a reactivepolymerization initiator. It is inert to polymerization and does notinterfere with the molecular weight control of the next polymerizationbatch as alcohol can.

It is usually advisable to contact and vigorously mix the gas with thepolymerization solution at the end of the polymerization reaction. Thiscontact and vigorous mixing can be effected by adding the hydrogen gasthrough spargers in a mixing vessel containing polymer solution. Thetime of contact should be at least about ten seconds and preferablyabout twenty minutes to allow sufficient contact time for the reactionto occur. This is dependent upon the efficiency of the gas contactingequipment, gas solubility, solution viscosity and temperature.Alternatively, a continuous system could be employed whereby hydrogen ispumped into a solution prior to going to a statically mixed plug flowreactor. Hydrogen could also be dissolved in an appropriate solution andadded to the polymer solution to be terminated. Another method would beto cause the hydrogen to be absorbed into an absorption bed an thencause the polymer solution to flow through the absorption bed. Thehydrogen contact could also be carried out by adding a material whichgives off hydrogen upon decomposition, i.e. diimide.

As stated above, the hydrogenation step of the present process iscarried out in the presence of a bis(cyclopentadienyl)titanium compoundof the formula set forth above. The hydrogenation step is carried out inthe absence of hydrocarbon lithium and alkoxy lithium compounds.Specific bis(cyclopentadienyl) compounds which may be used are describedin U.S. Pat. No. 5,291,990.

This process will selectively hydrogenate conjugated diolefins withouthydrogenating alkenyl aromatic hydrocarbons to any degree. Hydrogenationpercentages of greater than 50% are easily obtained but it has beenfound that in order to achieve hydrogenation percentages of greater than95% as is often desired, for many polymers the alkali metal hydride (forexample, lithium) to titanium ratio must be at least about 6:1 and canbe up to about 30:1. There has to be sufficient alkali metal hydride toensure quick and sufficient interaction between the two metals. However,since the amount of alkali metal hydride, usually lithium, is fixed bythe amount needed for polymerization, often times there is too muchhydride produced during the termination step and it is advantageous toadd another reagent to react with the alkali metal hydride to boostcatalytic activity.

Generally, polymers with molecular weights of less than about 100,000(and possibly higher if the solids content of the polymer solution ishigh) may very well have excess alkali metal (lithium) hydride presentafter termination. Since the alkali metal (lithium) hydride:titaniumratio is important in this process, the amount of titanium that is addeddetermines the amount of excess alkali metal (lithium) hydride as wellas polymer molecular weight and concentration of polymer in solution.The ratio can be no less than about 6:1.

The terminated polymer, preferably in solution, is treated with one ofthe following reagents. The reagents may be selected from the groupconsisting of R_(x) SiX_(4-x) where X is a halogen and x is 0-3,including silicon tetrachloride, difluorodiphenyl silane,dimethyl-dichloro silane, and silicon hexachloride; alcohols, includingmethanol, ethanol, isopropanol and 2-ethyl-1-hexanol; carboxylic acidsincluding 2-ethyl-1-hexanoic acid; phenols including 4-methyl-phenol;water; and halogen-containing hydrocarbons including dirbromethane.Generally, only a small amount of the reagent is need to react with thelithium hydride in the polymer solution to effectively boost catalystactivity and increase hydrogen conversions. These reactions are rapidwhich helps to eliminate any delay in process cycle time.

The polymer must be treated with at least some of the reagent but notmore than that amount which will achieve an alkali metal (lithium)hydride to titanium ratio of 6:1. If more of the reagent is used and thealkali metal (lithium) hydride to titanium ratio is lowered, thehydrogenation conversion falls off. Generally, this means that the molarratio of the reagent to the titanium in the polymer should not be morethan about 2:1 and it probably can be much less depending upon theequivalents of reactive sites available on the reagent molecule. Forinstance, dibromoethane appears to have two reactive sites whereassilicon tetrachloride appears to have four reactive sites, i.e. thehalogen atoms may be removed and replaced by hydrogen. Depending uponthe reagent used, it may be that not all of the reactive sites aresterically available or reactive such as in the case of silicontetrachloride. As the molecular weight of the polymer increases, lessreagent will be needed to achieve the advantages of the presentinvention and as the molecular weight goes down, more reagent will beneeded. The reason for this is that there is less lithium initiatorneeded per weight of polymer as the molecular weight of the polymerincreases. This results in less lithium hydride per polymer weightfollowing hydrogen termination of the polymerization.

For example, in a 50,000 molecular wight styrene-butadiene-styrene blockcopolymer (20% wt. polymer in solution) made using hydrogen termination,the lithium hydride level was determined to be about 33 ppm, solutionbasis. Silicon tetrachloride was the reagent of choice and it was usedin an amount equal to 100 ppm, solution basis. The molar ratio of LiH:Tiwas 11:1 and that of SiCl₄ :Ti was 2.0:1. The hydrogenation using thesilicon tetrachloride increased to 96% conversion of the olefinic doublebounds versus 92% conversion without the silicon tetrachloride.

In general, the hydrogenation is carried out in a suitable solvent at atemperature within the range of from about 0° to about 120° C.,preferably about 60 to about 90° C., and a hydrogen partial pressurewithin the range from about 1 psig to about 1200 psig, preferably fromabout 100 to about 200 psig. Catalyst concentrations within the rangefrom about 0.01 mM(millimoles) per 100 grams of polymer to about 20 mMper 100 grams of polymer, preferably 0.04 to 1 mM catalyst per 100gramsper polymer, are generally used and contacting at hydrogenationconditions is generally continued for a period of time within the rangefrom about 30 to about 360 minutes. Suitable solvents for hydrogenationinclude, among others, n-heptane, n-pentane, tetrahydrofuran,cyclohexane, toluene, hexane and benzene. Because of the small amount ofcatalyst present in the polymer after hydrogenation, it is not necessaryto separate the hydrogenation catalyst and catalyst residue from thepolymer. However, is separation is desired, it may be carried out usingmethods well known in the prior art. Hydrogenation may be carried out inother manners such as batch processes, continuous processes, andsemi-continuous processes.

EXAMPLES

A 600 lb. batch of polystyrene-polybutadiene-polystyrene (S-B-S⁻ -Li⁺)block copolymer 50,000 molecular weight (total peak MW determined byGPC) was made by anionic polymerization using sec-butyllithium as theinitiator in a 150 gallon pressurized reactor. The polymerization tookplace in a mixture of cyclohexane and diethyl ether. The resultingpolymer solution contained 20% polymer by weight. At the end of thepolymerization reaction, the reactor temperature was approximately 60°C. The reactor was sparged with hydrogen for approximately 20 minutes toterminate the polymerization.

Various amounts of reagents as described in Table 1 were added topolymer solutions prior to hydrogenation and titanium catalyst addition.The experimental hydrogenation runs consisted of pressure transferringto a 4-liter reactor 1560 g of a 20% by weight solution of polymer. Thissolution contained 6,5 mM of LiH or 33 ppm LiH, solution basis. Thereagents to promote activity were added to the polymer solution in theamounts shown in the table below. The temperature of the reactor wasmaintained at 75° C. At this point, 0.125 g or 0.5 mM of the catalyst,bis(cyclopentadienyl)titanium dichloride (Cp₂ TiCl₂), was added to thereactor as a toluene or cyclohexane slurry. If no reagent were added,the LiH:Ti molar ratio would be 13:1. After addition of the catalyst,the reactor was pressurized to 140 psig with hydrogen gas. The reactionwas allowed to run for 3 hours, during which time samples were drawnfrom the reactor and analyzed by proton NMR to determine final percentconversion of olefin. Gel Permeation Chromatography (GPC) was done onfinal samples to determine if there had been any changes in moleculararchitecture.

    __________________________________________________________________________                Amount ppm,    LiH:Ti                                                                            Conversion                                                 Solution                                                                             Reagent:Ti                                                                            Molar                                                                             at 3 hours                                     Reagent     Basis  Molar Ratio                                                                           Ratio                                                                             (%)                                            __________________________________________________________________________    Control-No Reagent                                                                         0     0       13  92                                             Dibromoethane                                                                             64     1.1     12  95                                             Dibromoethane                                                                             244    4.0      5  12                                             Silicon Tetrachloride                                                                     100    2.0     11  96                                             Difluorodiphenyl                                                                          122    1.7     11  99                                             Silane                                                                        2-ethyl-1-hexanol                                                                         50     1.2     11  96                                             2-ethyl-1-hexanoic                                                                        10     0.2     13  97                                             acid                                                                          Methanol     3     0.3     13  97                                             Water       50     9        4  91                                             __________________________________________________________________________

The results shown in the above table indicate that treatment of thepolymer solution with the reagents of the present invention increasesthe conversion of the olefinic double bonds when compared to the controlexperiment in which no reagent was added. We believe that the experimentperformed with 244 ppm of dibromoethane achieved poor results becausethe lithium hydride level was reduced too much, in essence lowering thelevel outside of the operable range for effective hydrogenation usingthis catalyst system or the reagent:Ti molar ratio was too high. Waterdecreased the LiH:Ti ratio below 6:1 and adversely affected the %conversion. It is believed that the reason is that the amount of waterused was much too high. Lower levels should result in improvedconditions.

We claim:
 1. In a process for the hydrogenation of conjugated diolefinpolymers which comprises:(a) polymerizing or copolymerizing at least oneconjugated diolefin with an organo-alkali metal polymerization initiatorin a suitable solvent thereby creating a living polymer an alkali metalhydride, (b) terminating the polymerization by the addition of H₂ priorto hydrogenation, and (c) selectively hydrogenating the unsaturateddouble bonds in the conjugated diolefin units of said terminated polymerby contacting the polymer, in the absence of hydrocarbon lithium andalkoxy lithium compounds, with hydrogen in the presence of at least onebis(cyclopentadienyl)titanium compound of the formula: ##STR2## whereinR₁ and ₂ are the same or different and are selected from the groupconsisting of halogen groups, C₁ -C₈ alkyl and alkoxy groups, C₆ -C₈aryloxy groups, aralkyl, cycloalkyl groups, silyl groups and carbonylgroups; the improvement which comprises treating the terminated polymerwith a small but effective amount of a reagent which will react toreduce the amount of alkali metal hydride in an amount which willdecrease the alkali metal hydride to titanium ratio in the terminatedpolymer to no less than 6:1 to boost catalyst activity and increasehydrogen conversion.
 2. The process of claim 1 wherein the reagent isadded such that the molar ratio of reagent to titanium is no more thanabout 2:1.
 3. The process of claim 1 wherein the polymer has a molecularweight of less than about 100,000.
 4. The process of claim 1 wherein thereagent is selected from the group consisting of R_(x) SiX_(4-x), whereX is halogen and x is 0-3, silicon hexachloride, alcohols, phenols,carboxylic acids, water and halogen-containing hydrocarbons.
 5. Theprocess of claim 4 wherein the reagent is selected from the groupconsisting of silicon tetrachloride, difluorodiphenyl silane, dimethyldichlorosilane, methanol, ethanol, isopropanol, 2-ethyl-1-hexanol,2-ethyl-1-hexanoic acid, 4-methyl phenol, dibromoethane and water. 6.The process of claim 1 wherein the hydrogenation is carried out at atemperature from about 0° C. to about 120° C. and a pressure of fromabout 1 psig to about 1200 psig and the catalyst concentration is fromabout 0.01 mM to about 20 mM of titanium per 100 g of polymer and thecontacting takes place for a period of time within the range from about15 to about 1440 minutes.
 7. The process of claim 1 wherein the alkalimetal initiator is an organo lithium compound.
 8. The process of claim 7wherein the organo lithium compound is sec-butyllithium.
 9. The processof claim 1 wherein the titanium compound is selected from the groupconsisting of bis(cyclopentadienyl)titanium dichloride,bis(cyclopentadienyl)titanium dibromide, bis(cyclopentadienyl)titaniumdiiodide, bis(cyclopentadienyl)titanium difluoride,bis(cyclopentadienyl)titanium dicarbonyl, bis(cyclopentadienyl)titaniumdimethyl, bis(cyclopentadienyl)titanium diethyl,bis(cyclopentadienyl)titanium dibutyl, bis(cyclopentadienyl)titaniumbis(trimethylsilylmethyl), bis(cyclopentadienyl)titanium dibenzyl,bis(cyclopentadienyl)titanium dihexyl, bis(cyclopentadienyl)titaniumdimethoxide, bis(cyclopentadienyl)titanium diethoxide,bis(cyclopentadienyl)titanium dibutoxide, bis(cyclopentadienyl)titaniumdipentoxide, bis(cyclopentadienyl)titanium dineopentoxide,bis(cyclopentadienyl)titanium dephenoxide and all mixtures thereof. 10.The process of claim 9 wherein the titanium compound isbis(cyclopentadienyl)titanium dichloride.
 11. The process of claim 1wherein the alkali metal hydride:titanium metal ratio during thehydrogenation is at least 6:1.
 12. The process of claim 1 wherein theconjugated diolefin is selected from the group consisting of butadieneand isoprene.
 13. The process of claim 1 wherein the diolefin iscopolymerized with a vinyl-substituted aromatic hydrocarbon.
 14. Theprocess of claim 13 wherein the diolefin is copolymerized with a monomerselected from the group consisting of styrene and styrene derivatives.15. The process of claim 14 wherein the copolymer is a block copolymerhaving at least one conjugated polymer block and at least one styrene orstyrene derivative block.
 16. The process of claim 1 wherein at least95% of the unsaturated bonds in the conjugated units are hydrogenated.